Acoustic spectral preferences in two cryptic species of grey treefrogs: implications for mate choice and sensory mechanisms

Females are expected to use spectral properties of acoustic signals for mate choice because dominant frequency often provides reliable information about male size. The advertisement calls of grey treefrogs (Hyla chrysoscelis and H. versicolor) have two main frequency bands: the amplitude of the dominant, high-frequency band averages 7-10 dB greater than that of the low-frequency band. The frequencies of these peaks were negatively correlated with male body size in both species. I used two-speaker playbacks of synthetic calls to generate preference functions for both species based on simultaneous variation in the frequencies of the two bands and, for H. versicolor, on variation in single-component calls; frequencies of most of these stimuli fell within the range of the dominant, high-frequency band. When two-component sounds were used, preference functions were bimodal and broad, with peaks close to (H. versicolor) or slightly higher than (H. chrysoscelis) mean frequencies in conspecific calls. Because preferences were not strong, I predict that females exert only weak stabilizing or directional selection on male call frequencies. When single-component calls were used in tests of H. versicolor, the preference-function peak was shifted to a lower-than-average frequency within the high-frequency range. I hypothesize that such a frequency may better stimulate both of the inner ear organs, whose tuning roughly matches frequencies in the two bands in conspecific calls, than a single component of average or much lower-than-average frequency. Females of both species preferred calls with both spectral peaks to calls with a single, high-frequency peak, even when the low-frequency peak was attenuated by 24-30 dB. This result serves to emphasize that typically subdominant frequency components can have seemingly disproportionate effects on signal attractiveness.